1. Field of the Invention
The invention relates to a piston, having a head with a geometry such that it affords an active guiding action, for an internal combustion engine, in particular of the type operating with controlled ignition and direct fuel injection.
The invention also relates to a combustion chamber co-operating with a piston as proposed by the invention, for an internal combustion engine having controlled-ignition and in which fuel is injected directly into the combustion chamber.
More specifically, the piston proposed by the invention delimits a cooperating combustion chamber with a cylinder in which said piston is axially slidable, and a cavity provided in a cylinder head, facing the piston head, joined to each of the engine cylinders, the piston head having an end face directed towards the combustion chamber and which has, inside a planar peripheral ring extending perpendicular to the axis of the piston, firstly, a bowl-shaped concave recess open towards the combustion chamber and disposed eccentrically on one side of the piston axis and, secondly, a boss projecting into the combustion chamber out from the peripheral ring and eccentrically disposed on the other side of the piston axis, said recess and boss being symmetrical relative to a diametric direction of the piston head, as disclosed by FR 2 757 211.
2. Description of the Prior Art
It is generally known that various geometries have recently been suggested for the end faces of piston heads in internal combustion engines and hence also for the associated combustion chambers, in order to improve the process of preparing the air/fuel mixture with a view to reducing simultaneously the fuel consumption of these engines and the pollution which they create through their combustion gases.
To this end, it has been proposed that operation of the engine, particularly when used at low load, be controlled with what is known as a poor air/fuel mixture (in which the proportion of air is greater than the stoichiometric proportion), which may be homogeneous or stratified, particularly if it is outside the flammability limits of the homogeneous mixture, whereas when the engine is operating at medium or high loads, the latter should be supplied with a fuel mixture in a stoichiometric proportion, the mixture being rich only under certain particular engine operation conditions, particularly certain transitory conditions or when cold-starting, for example.
Irrespective of the richness of the fuel mixture supplying the engine, the combustion characteristics of this mixture in a combustion chamber will depend on numerous factors, including geometric factors, relating to the shape of the combustion chamber and hence the geometry of the end face of the corresponding piston head (and, therefore the shape of the recess and boss in the head of a piston as described above), aerodynamic factors, and in particular the velocity and turbulence of the liquid fuel jet, of the air sucked into the cylinder and of the gaseous flow inside the combustion chamber, as well as dimensional and geometric factors pertaining to the injector, such as the number, the shape and dimensions of the injection orifices, the internal structure and the pressure delivering fuel to the injector.
It is evident that optimisation of the internal aerodynamics of a combustion chamber, in particular at the timing of the air admission, fuel injection and ignition of the combustible mixture, are crucial design factors for an engine with controlled-ignition and direct injection, especially in terms of their contribution to preparing as an efficient as possible fuel mixture, be it stoichiometric or poor, in a region of the combustion chamber which is conducive to good combustion (good flammability and controlled propagation of the flame front in combusting the mixture).
The geometry of the combustion chamber is specifically designed to promote certain aspects of the mixture-forming process, in particular the global structure of the aerodynamics, such as what is referred to as the swirl flow describing a rotary macro-structure about the axis of the combustion chamber and what is referred to as the tumble flow describing the axial motion which enables the deflection of the fuel jet or jets through the main flow to be characterised, thereby providing a better evaluation of the distribution of the fuel and its vaporisation in the combustion chamber. At the end of the compression phase, the geometry must also be conducive to promoting a certain degree of turbulence in the fuel charge so that the parameters governing exchange of mass in the vicinity of the flame front can be controlled locally, thereby determining the combustion speed.
In order to improve distribution and vaporisation of the fuel in the combustion chamber as well as the air-fuel mixture with a view to improving combustion, different geometries specific to the combustion chamber have been proposed recently, in which one or more concave bowl-shaped recesses are provided in the end face of the piston head and open towards the combustion chamber, the injector directing the fuel jet into one of these concave recesses which deflects this jet towards the centre of the combustion chamber facing the ignition plug, whilst it is simultaneously atomised into fine droplets of fuel and/or vaporised and mixed with the air admitted to the combustion chamber by means of the flow and turbulence modified therein due to the presence of the concave recess or recesses.
By way of example, DE 197 13 030 describes a four-stroke internal combustion engine, with controlled ignition and direct injection, in which the end face of a piston head delimiting a combustion chamber has an arrangement of substantially H-shaped guide ribs projecting towards the combustion chamber and comprising two longitudinal ribs, parallel and at a distance apart, perpendicular to the longitudinal plane of the engine intersecting the axes of the cylinders thereof, and joined to one another by a transverse rib, parallel with the longitudinal plane of the engine and substantially perpendicular to the longitudinal ribs. Designed to optimise the degree of turbulence occurring locally at the end of the compression phase and amplify the aerodynamic tumbling motion of the gas admitted to the cylinder with a view to improving combustion by specifically acting on the speeds and distribution of the fuel and combustive air, these ribs delimit, between them and with the peripheral planar ring of the piston head, four concave bowl-shaped recesses which are open towards the chamber. One of the two central recesses (between the H-shaped ribs) receives the fuel jet from an injector mounted in the side of the cylinder head, in which the cavity delimiting the combustion chamber is a roof-shaped cavity with two inclined faces, the injector opening into the base of one of the two inclined faces between two admission valves co-operating with the admission ports in this inclined face, the axis of the injector being inclined relative to the axis of the piston and combustion chamber and contained in a radial plane intersecting this axis, with which the plug mounted in the cylinder head is aligned at the top of the combustion chamber.
In DE 197 13 029, the piston head has, on its end face directed towards the combustion chamber, a structure of T-shaped ribs projecting towards the combustion chamber and which delimit in this end face, in conjunction with the planar peripheral ring of the piston head, three concave bowl-shaped recesses open towards the combustion chamber, one of which receives the fuel jet from the injector, having an inclined axis relative to the axis of the piston and plug and directed towards this axis, but arranged in the apex of the combustion chamber at one end of the join between the two inclined faces delimiting the roof-shaped cavity in the piston head to form the corresponding part of the combustion chamber.
In U.S. Pat. No. 5,727,520, the end face of the piston head has a very open, U-shaped guide rib projecting towards the combustion chamber, delimiting two concave recesses open towards the combustion chamber, the one located inside the U receiving the fuel jet from an injector centrally disposed in the top of the roof-shaped combustion chamber with two inclined faces, the injector being slightly inclined relative to the axis of the piston and combustion chamber, whilst the plug is inclined still further relative to this axis and disposed to the side in one of the two inclined faces of the roof-shaped cavity of the cylinder head, on the admission side, the U formed by the ribbing in the piston head being open towards the exhaust.
In these various embodiments, the ribs bounding the concave recesses on the piston head are essentially designed to modify the flow with a view to amplifying the tumble motion whilst promoting local turbulence so as to enhance the quality of the air/fuel mixture, its flammability and its combustion speed, the aim generally being to improve combustion in order to reduce consumption and pollution.
The underlying purpose of the invention is to improve performance in terms of consumption and pollution even more than the designs disclosed in the above-mentioned patents, by improving the level of local turbulence still further at the end of compression phase as well as the distribution and vaporisation of the fuel in the combustion chamber by means of a combined strategy of aerometric guidance and guidance by means of a solid wall, making the geometry of the combustion chamber more conducive to producing a good preparation of the directly injected stratified poor mixture in particular, in order to guarantee efficient combustion, depending on the position of the plug, and in order to create an air/fuel mixture in stoichiometric proportions in the central volume of the combustion chamber facing the plug.
Another objective of the invention is to achieve the above-mentioned improvements by perfecting a piston head of the type known from FR 2 757 211, the end face of which, bounding the combustion chamber, has, inside a planar peripheral ring, perpendicular to the axis of the piston, firstly, a bowl-shaped recess with a concave base open towards the combustion chamber and eccentrically disposed on one side of the piston axis, and, secondly, a boss projecting towards the combustion chamber out from the peripheral ring, and eccentrically disposed on the other side of the piston axis, the recess and the boss being symmetrical relative to a diametric direction of the piston head.
To this end, the piston with an active guiding head, of the abovementioned type and disclosed in FR 2 757 211, proposed by the invention, is characterised in that the boss, seen in a plan view, parallel with the piston axis, is of a substantially horseshoe shape partially surrounding said recess, which is also surrounded by an arc of said planar peripheral ring, and in that the boss has an overhanging rim above the join of said recess to said boss, at least on a level with the base of the substantially horseshoe-shaped boss and at the parts of wings of the horseshoe-shaped boss adjacent to said base.
Accordingly, the shape of the combustion chamber and more specifically the shape of the recess and the boss on the piston is such that it creates turbulence conducive to forming a good air/fuel mixture, whilst allowing the jet of fuel from the injector to be deflected towards the centre of the combustion chamber, thereby producing better atomisation and vaporisation of the fuel.
Advantageously, in addition, the overhanging rim does not have a sharp edge but, on the contrary, is a rim with a rounded cross section and, what is more, the amplitude of the overhang of said rim, viewed in a plane perpendicular to the piston axis, is gradually attenuated or less pronounced from the centre of the base of the horseshoe boss towards the end of each of the two wings of said boss, on a level with which ends the overhang is virtually zero.
This technical feature reduces or even obviates the need for devices that would otherwise be necessary upstream within the air supply circuit to create a swirling motion of the gas in the combustion chamber, these upstream devices being complex and expensive.
Moreover, the depth of the recess, viewed parallel with the piston axis, is advantageously variable and is deeper in a part of the recess which is not adjacent to said base of said horseshoe-shaped boss, this maximum depth as well as, more generally, the profile of the recess bottom preferably depending in particular on the diameter of the piston as well as the angular position of the injector relative to the axis of the piston and combustion chamber.
Advantageously, moreover, the horseshoe-shaped boss has a convex face directed towards the combustion chamber and with a radial width, relative to the piston axis, which is at its maximum in the diametric direction of symmetry of the recess and boss on the piston head and progressively decreases on either side of this direction, into the wings of the horseshoe-shaped boss. As a result of this advantageous feature, the internal volume of the combustion chamber occupied by a gaseous mixture exhibiting poor conditions of flammability and mixture, can be reduced.
Simultaneously, the planar peripheral ring has a variable radial width, being at its minimum on said diametric direction of symmetry of the recess and boss, and, on the side located after said boss relative to said recess, the radial width of the peripheral ring progressively increases on either side of its section of minimum width out to the sides of the wings of the horseshoe-shaped boss, so that this shape of boss in conjunction with this enlargement of the planar peripheral ring facilitates a lateral sweeping of the unburned gases, in particular towards the central combustion zone.
The invention also relates to a combustion chamber, for an internal combustion engine with controlled ignition and direct injection, delimited between a piston head, axially slidable in a cylinder of the engine, and said cylinder as well as a cavity arranged facing said piston head in a cylinder head joined to said cylinder, an ignition plug being provided in the cylinder head substantially coaxial with the piston, and a fuel injector also being provided in the cylinder head to deliver at least one fuel jet into the combustion chamber directed along an inclined axis relative to the axis of the piston, the combustion chamber proposed by the invention being characterised in that the piston is a piston as proposed by the invention and defined above, the inclined axis of the fuel jet being in a substantially radial plane intersecting the piston axis and said diametric direction of symmetry of said recess and boss of the piston head, said fuel jet being directed into said recess and towards said overhanging rim of the base of said horseshoe-shaped boss, in order to produce an excellent active guiding action.
In addition, the combustion chamber is advantageously of a known high-performance type, in which the corresponding cavity of the cylinder head is a substantially roof-shaped cavity with two inclined faces, into one of which there open said injector and at least one air admission duct selectively closed and opened by at least one admission valve, substantially facing said recess of the piston head, whilst at least one exhaust duct, selectively closed and opened by at least one exhaust valve, opens into the other of said inclined faces substantially facing said boss of the piston head.
Excellent performance has been achieved in a combustion chamber of this type with an injector inclined relative to the piston axis so that the axis of said fuel jet subtends an angle of between approximately 80xc2x0 and approximately 45xc2x0 with the piston axis.